The invention relates to methods for determining the permeability of hydrocarbon-bearing formations through the measurement of fluorescence from formation samples.
The permeability of material, such as rock found in hydrocarbon formations, is a very desirable piece of information to geologists and drilling engineers. While all soils and rock are permeable, the degree of permeability is highly variable. The measurements of permeability of a soil or rock indicate how well a fluid will flow through the pores of the material.
It has been known for some time to measure the permeability of core samples and drilling mud removed from hydrocarbon-bearing formations.
U.S. Pat. No. 4,253,327 to Phillips Petroleum Company in 1981 describes the use of elevated pressure and temperature to determine the permeability of a core sample. A core sample is heated and a fluid is supplied under a first desired pressure and allowed to flow through the sample. A second pressure is applied to the surface area of the sample and the pressure measured of the fluid flowing to the first and second face of the sample. The permeability is calculated based on the pressure differential across the core sample.
U.S. Pat. No. 5,133,207 to Wilson et al. describes a complex system of instrumentation designed to measure the permeability of multiple core samples in which a fluid under pressure is provided along with a plurality of permeameters each having an inlet and an outlet and a means for measuring the amount of fluid passing through the core sample in each permeameter.
The above techniques require elaborate sample preparation and are typically performed off the drilling site. Further, Applicant is unaware of any permeability tests for cuttings samples from returned drilling mud.
It would be advantageous to have a method of determining permeability of a hydrocarbon-bearing formation that minimizes sample preparation and can be performed on site, thereby providing realtime permeability information to the drilling engineer.
Fluorescence techniques are also known, which are simply to shine a broad spectrum ultraviolet light on a sample of cuttings, fluorescence being a crude indicator of the presence of oil. Techniques are also known for testing drilling mud samples for the presence of hydrocarbons using these fluorescent techniques.
U.S. Pat. No. 2,361,261, to Campbell describes a method of determining whether an oil producing zone has been penetrated by comparing fluorescence from drilling fluids samples discharge from the well to that entering the well. The samples are subjected to UV light and the fluorescence measured using conventional instrumentation which provides a reading on a galvanometer which can be converted into terms of oil by comparing to samples of standard drilling fluid with known amounts of oil added. Campbell teaches separating out and removing the coarse particles. A small amount of solvent may be added to the sample however, it is not added to extract oil from the main body of the sample, but merely to concentrate surface oil. Campbell does not contemplate utilizing the methodology to provide information regarding sample porosity and permeability.
U.S. Pat. No. 2,435,843 to Rand describes a method for examining well cuttings for the presence of hydrocarbons. A sample of cuttings is placed in a shallow dish containing sufficient non-fluorescing solvent to cover the sample which is then exposed to an ultraviolet light. An observer visually examines the same for release of plumes of fluorescence released from the sample. Rand""s method does not teach continued immersion of the sample in the solvent, but instead requires an immediate examination when the sample is placed in the solvent. Rand""s method relies upon qualitative observation and subjective comparison on the part of the observer to provide data regarding time a vigor of plume formation to determine porosity and permeability of samples. While Rand suggests that logs can be made of the color, number and vigor of the plumes observed from samples taken at various depths to be used as a useful adjunct to logs of other types, he does not teach any means of quantitative measurement, nor does he teach a method of standardizing observations between samples to provide standardized comparative data.
Another such technique is described in U.S. Pat. No. 4,990,773 to Supernaw et al. which involves testing for hydrocarbon producability by determining the amount of highly viscous asphaltenes present in drilling mud samples. A sample solvated in a polar solvent which solvates asphaltenes is compared to a sample solvated in an aliphatic solvent which solvates most crude fractions without solvating the asphaltenes. The solvates are exposed to UV light at approximately 254 nm at which wavelength most petroleum compounds fluoresce. The amount of fluorescence emitted is measured at 320 nm and a ratio between the two samples is used to determine the producability of the hydrocarbon formation.
Further, in U.S. Pat. No. 4,977,319 to Supernaw describes a similar technique used to determine the hydrocarbon content of an underground formation. The sample is simply solvated with a solvent which readily solvates hydrocarbons. The solvate is quantitatively measured using a fluorometer with an excitation wavelength below 400 nm. Most petroleum compounds fluoresce at this excitation wavelength. The results are compared to previous data obtained comparing intensity against a predetermined correlation of fluorescence and oil fraction. As disclosed, Supernaw and the prior art identified therein teach the measurement of fluorescence intensity at one or more wavelengths.
The current invention builds on the known techniques of solvation of hydrocarbons and measurement of emitted fluorescence to expand the known measurement techniques to now include determination of permeability of drilling mud samples.
A method is provided for testing drilling mud and establishing a measure of the permeability of the cuttings contained therein.
In a preferred embodiment, cuttings sediment is centrifuged from drilling mud, heated to drive off surface hydrocarbons and is subjected to light for fluorescence excitation. A non-fluorescent diluant is added to the sediment and the fluorescence emission is measured. Dependent upon the permeability of the sediment, the speed at which the fluorescence brightness develops is found to vary. Specifically, both the time and brightness of the emission are monitored. The time/brightness relationship is proportional to the permeability of the cuttings. While the brightness and the permeability can be estimated by the experiment eye, it is preferable and more reliable to use apparatus capable of providing reproducible, standardized fluorescence measurement. Preferably, a blank is used to calibrate the background brightness of the diluant alone or a detect a diluant which has been contaminated.
Having determined brightness measurements over time, a fluorescence index can be calculated and used, along with other known parameters such as grain size, angularity, sortability and a porosity factor, to calculate a permeability index.
In a broad aspect, a method of semi-quantitatively identifying the presence of hydrocarbons filling the pores of cuttings samples from a drilled formation is performed, the hydrocarbons being fluorescence when exposed to UV light, and comprising the steps of;
adding a non-fluorescent diluant to the sample;
exciting the sample;
measuring elapsed time;
measuring the intensity of the fluorescence emission as a function of the elapsed time for establishing a relationship for that sample; and
establishing a numerical value for a representative brightness of the fluorescence emission as a function of the elapsed time (FI).
Accordingly, in another aspect of the invention, a method is provided for quantifying a fluorescence index (QFI) for a sample from a drilled formation comprising the steps of:
assigning a numerical value to the percentage of a sample that fluoresces when excited for establishing a quantity factor QTY; and
determining a quantitative fluorescence index for the sample being substantially proportional to QTY and FI.
Accordingly, in another aspect of the invention, a method is provided for quantifying a permeability index (PI) for a sample from a drilled formation comprising the steps of:
assigning numerical values for the proportion grains in each of a plurality of grain size divisions within a sample and a weighting factor for each division corresponding thereto for establishing a environmental index value EnviroNdx;
assigning a numerical value to the degree of angularity of grains within a sample for establishing an angularity index value AngNdx;
assigning a numerical value to the degree to which the grains within a sample are the same for establishing a sorting index value SrtNdx;
assigning a numerical value to the degree of porosity of the sample for establishing a porosity value Por; and
determining the relative index PI of the sample as being substantially proportional to QFI EnvNdx, AngNdx, SrtNdx, and Por.
Most preferably the relationship is as follows:
PI=QFIxc3x97EnvNdxxc3x97AngNdxxc3x97SrtNdxxc3x97Por